72 PATHOGENIC BACTERIA. 



II. The Subject of Infection. — i. Natural and Ac- 

 quired Immunity. — The peculiar natural and acquired 

 conditions of resistance to disease which constitute im- 

 munity are so interesting and important that a separate 

 chapter has been devoted to their discussion, wherein the 

 student will fiud much that will merit thoughtful atten- 

 tion. 



Immunity according to its degree will, of course, make 

 infection impossible or difficult; and except in cases of 

 extreme virulence of the bacteria immune animals do 

 not become infected. In the rare cases in which infection 

 does occur it is frequently so modified that the symptoms 

 and lesions vary considerably from the usual type. 



2. Vital Condition. — In susceptible animals the degree 

 of susceptibility may vary greatly with conditions that 

 arise. In some diseases the susceptibility is more marked 

 in youth, in others in advanced age ; the existence of 

 constitutional diseases, as diabetes, markedly predisposes 

 to infection. Sex is sometimes a predisposition to infec- 

 tion, in that organs possessed by one sex, and not by the 

 other, readily become infected; thus, women and not men 

 suffer from puerperal infections. 



3. Injury. — Any damage or injury to the body favors 

 the lodgement and activity of whatever bacteria may 

 reach the damaged area. It seems to be a well-established, 

 though not perfectly comprehensible, fact that tuberculosis 

 of the bones and joints of children is determined by an 

 injury of the part. The existence of a slightly damaged 

 cardiac valve is frequently the nidus in which circulating 

 bacteria find rest and produce the malignant or ulcerative 

 endocarditis so frequent in the infectious diseases. 



Rosenbach was able to prove the predisposition of 

 damaged tissue to bacterial invasion by injuring a cardiac 

 valve by a sound passed into the aorta, and then inject- 

 ing a culture of staphylococci. These micro-organisms 

 operating upon the diseased valve produced malignant 

 endocarditis. 



Sources of Infection. — The experiments of Nuttall 



